"nobody has challenged Coulter's legal right to say whatever she pleases"
I think there's something you might not be looking at.
The left are trying to create a social climate that will both prevent people from speaking their minds by intimidating them, and groom the public to accept legal curtailment of our First Amendment rights.
Wherever the left is at work, our Constitution is under attack, even if they are yet one step away from legal challenges.
That's all true, but this still isn't a first amendment issue, as nobody challenged her legal rights, only the nature of her comments.